1. Technical Field
The invention relates to image sensor systems and, in particular although not exclusively, to active pixel CMOS image sensors implementing an electronic global shutter.
2. Background Art
Digital image capturing devices use image sensors to convert incident light energy into electrical signals. An image sensor includes a two-dimensional array of light sensing elements called pixels. For example, a CMOS image sensor includes an n×m array of photosensitive structures (for example, photodiodes) as well as circuit elements for readout and control (a photodiode plus its immediate readout and control transistors are typically called a “pixel”). Each pixel in the array produces an electrical signal corresponding to an amount of light incident upon the pixel during an integration period. Outputs from the light sensing elements are converted to digital form and stored digitally to form raw data representing a scene. The raw data can be processed by an image processor to produce rendered digital images. Image sensor designs include Charged Coupled Devices (CCD), Complementary Metal Oxide Silicon (CMOS) image sensors, Digital Pixel System (DPS) sensors, and the like. CMOS image sensors are advantageous in that they consume a lower level of power.
Active pixel CMOS image sensors are designed to include control elements (e.g. MOS transistors) at each pixel for controlling photon integration in the photodetector, controlling reset, and providing a conversion gain to the pixels. Active pixel CMOS image sensors can support either rolling shutter or global shutter modes, or both.
In both rolling shutter and global shutter modes, the signal charge generally sits on a floating diffusion FD (described further below) until the row is read out. In rolling shutter mode, signal charge only waits on the floating diffusion for several microseconds before being read out. In global shutter mode, however, this time can be easily several milliseconds. During the time the floating diffusion is waiting to be read out, it can collect substantial dark signal, as the floating diffusion dark current is typically quite high (1000-2000 e−/s at 60 C). This dark signal increases FPN and temporal noise. Additionally, when the signal charge is sitting on the floating diffusion waiting to be read out, the next frame is being integrated in the photodiode, and light from this next frame can distort the image of a current frame if the light reaches the floating diffusion prior to the signal being read out.
Accordingly, the present disclosure provides for an image sensor in which dark current flowing to a floating diffusion can be reduced and light can be prevented from reaching the floating diffusion.